Chem 241 Sample Questions Exam #3
Transition Metal Bonding
1. Draw the MO diagram for a simple donor acceptor interaction. Label everything. Label the donor as D:
and the acceptor as A.
2. Draw the MO diagram for the octahedral transition metal complex Ni(NH3)62+. Enter electrons. NH3 is
a sigma donor ligand, but has no pi interactions with the metal.
3. Determine the bond order for Ni(NH3)62+ and for Cr(NH3)62+. They have the same MO diagram but
differ in their number of electrons.
2 and 3 answers:
4. For which d-electron configurations are different low- and high-spin cases possible in octahedral
complexes?
d4, d5, d6, d7
5. Draw the MO diagrams for square planar and for tetrahedral 4-coordinate complexes. Determine the
bond order for both with each possible d-electron configuration (0 to 10 d electrons).
Which structure is preferred in each case?
When there is a tie in terms of bonding, which will prevail?
6. What type of transition leads to Ni(NH3)62+ having a green color?
Again, NH3 is a sigma-only ligand. If NH3 is replaced by oxalate ion (C2O42-) which is a pi-donor
ligand, will the absorption shift to higher or lower energy?
If NH3 is replaced by CO, which is a pi acceptor ligand, in which direction will the absorption
shift?
7. Which compound is more stable in terms of the ligands not falling off: Ni(NH3)62+ or Ni(en)32+?
Remember that “en” stands for ethylene diamine, which is a bidentate ligand that has similar bond
strength to the metal as NH3.
Ethylenediamine is more stable due to entropy effects (this is called the chelate effect).
8. Why are the following essentially colorless? Mn2+(aq), Zn2+(aq), Cr(CO)6
9. The formula for lattice energy, Uo, is:
What are each of the terms?
If you compare LiF and CsBr (which you can assume have the same structure), which terms will be larger
for LiF and which terms will be larger for CsBr?
Uo larger for LiF.
Z+ and Z- are the same for both because they have the same charges (+1 and -1).
M is the same for both because they have the same shape.
Ro is later for CsBr because of larger ion radii. This favors stronger lattice energy for LiF.
n is larger for CsBr because of the larger ions. (So, (1 – 1/n) is larger for CsBr, making the lattice energy
weaker.)
Which will have the stronger lattice energy? LiF, due to smaller ro and smaller n.
10. What’s this about? Explain it and what it tells us.
11. Which ion should have the greater hydration energy? Which the greater hydration number?
Li+, Na+, K+, Mg2+, Al3+
Both hydration number and hydration energy increase as Z2/r increases. So, the most important factor is
the charge, Z, and then the smaller the radius, the larger these properties.
Al3+ has the largest charge and smallest radius, so has the largest hydration energy and number.
K+ has the lowest of these.
12. Why is AlPO4 insoluble, but KNO3 is soluble? Explain in terms of enthalpy and entropy contributions.
This chart shows the relative effects; favorability is in terms of the salt being soluble.
Enthalpy of
separating ions
Enthalpy of
hydration
Entropy of ions
Entropy of water
AlPO4
Larger, strongly
disfavors
Larger, strongly
favors
Moderate,
moderately favors
Much larger,
strongly disfavors
KNO3
Smaller, weakly
disfavors
Smaller, weakly
favors
Moderate,
moderately favors
Small, disfavors
only slightly
Notes:
These two pretty much offset each
other. No enthalpy preference for either
salt.
This is about equal
for the two salts.
This term is the big
difference,
disfavoring
solubility of AlPO4.
13. Is SO3 acidic, basic, or neither? Give a reaction that shows why.
Is Na2O acidic, basic, or neither? Give a reaction that shows why.
SO3 is acidic- as are all nonmetal oxides. It is a Lewis acid and reacts with water to form sulfuric acid:
SO3 + H2O  H2SO4
Na2O is basic- as are all metal oxides. It reacts with water to form NaOH.
Na2O + H2O  2 NaOH
14. What reaction or reactions lead to the initial formation of caves? That is, what makes a hole in the
ground?
What reaction/reactions lead to the formation of, well, formations inside caves?
Cave formation occurs by two reactions: If there is acid in the water trickling into the CaCO3 rockbed,
then the reaction is:
H+(aq) + CaCO3(s)  Ca2+(aq) + HCO3-(aq)
{there are few variations on this theme}
If the water is passing through soil, it will be high in CO2. Then high CO2 concentration shifts equilibrium
A toward H2CO3. This H2CO3 reacts very favorably with CaCO3 to dissolve it to form HCO3-.(equilibrium B
shifts right)
When the cave opens to the outside air, the reverse occurs. CO2 pressure decreases; this shifts
equilibrium A away from H2CO3. The decrease in H2CO3 shifts equilibrium B to the left- causing HCO3- to
reform CaCO3(s).
15. What will result from a cave that forms, breaks the surface of the earth, and then a lot of time goes
by? Lots of time. What will you find when you go there?
I’m not answering this.
16. Describe how a simple Zn/Cu battery works. Describe each part of the cell and what chemically
occurs there.
17. Using your chart of reduction potentials, which battery would produce a higher voltage:
Zn/Ag or Zn/Cu
Zn/Ag does because it the potential difference is greater between Zn and Ag.
18. Describe the workings of: a. a lead-acid battery and b. an alkaline battery.
See your notes.
19. What is a fuel cell?
An electrochemical cell that essentially runs a normal “burning” reaction in a way that captures electrical
energy.
20. What does the technique cyclic voltammetry tell us about a compound?
Two things:
1. How difficult or easy it is to add or remove an electron to a molecule. It does this by the position of the
voltammogram wave.
2. Whether the product of oxidation or reduction is stable. It does this by observing whether the
voltammogram wave is “reversible.”